800 MHZ vs. 1900 MHZ

I am surprised they don't have more capacity problems with their bandwidth-handicapped network.

 

What helps them out is the number of people who use direct connect, they use( at least I believe they use) 6 time slots for direct connect and 3 for phone calls. Let me know if I am off here. There is a definite cap to their capacity and I think they are starting to get near it the way they are pushing for the freq swaps.

 

Motorola has developed a 4kHz vocoder for the iDen network, in effect potentially doubling Nextel's network capacity (they would have to migrate existing customers to new phones that use this vocoder and of course upgrade the basestation equipment to take advantage of the new vocoder)

 

Nextel digital two-way radio service uses a half-duplex signal. A normal cell phone call uses two separate frequencies, one to send and one to receive, for each call while a Direct Connect call uses only a single frequency. Each frequency uses 25khz of bandwidth which can carry 6 time slots.

Does moving to a 4kb vocoder mean they will be able to fit more time slots into the 25khz channel? or will they make the channel narrower and still have 6 time slots? In order to increase capacity they have to do either one of these.

 

After reading the technical doc for iDEN, it appears that instead of using the typical 2 timeslots of the six timeslots in a frame for an 8 kbs vocoder, they'll be able to use one time slot of six for the 4 kbs vocoder. This is my initial guess.

Excerpt from Moto Doc ( http://idenphones.motorola.com/iden/developer/downloads/techover.pdf )

iDenRadio Carrier Access Method:
To support and facilitate the use of resources by more than one conversation, the
iDEN system divides the radio link data stream by time.

Time Division Multiple Access:
Time Division Multiple Access (TDMA) allows more than one user or device to
multiplex on (share) a given carrier. In the iDEN system the radio carrierâ??s digital
data stream is divided by time (Figure 3-5). Since the data stream runs much faster
than is required during communication, other data or conversations can be placed on
a single radio carrier (may be interleaved) without degrading or interfering with each
other. This increases the possible conversations per radio from one (analog cellular)
to:
- 3 per radio (Interconnect)
- 4 per radio (2-Dispatch and 2-Interconnect)
- 6 per radio (Dispatch only)

This provides several benefits:
- Reduced base station costs as compared to analog transmissions
- Full-duplex support â?? allowing the MS to switch between transmit and receive.
- No incremental hardware to support dispatch, interconnect, and messages.

The iDEN TDMA system divides the RF carrier into 6 discrete timeslots of 15 ms in
duration. Each of these timeslots is a separate unit that contains; overhead for
transmitter turn-on, training & synchronization, propagation delay, or conversation.
Auxiliary data imbedded within each slot provides associated signaling. The sharing
of the timeslots (interleaving) increases the carriers capacity.
Timeslot 1 on a BR of each sector or cell is designated as a Primary Control Channel.
These channels are used for call setup, MS contact and MS location. Secondary
Control Channels may be assigned if traffic volume is high enough to saturate the
Primary Control Channels.

Timeslot Allocation:
A typical iDEN option increases radio link timeslots allocated to a single voice
conversation for interconnect calls from 1 to 2 per frame. By doubling the timeslots
available, the voice sampling rate can be increased. An increase the voice sample
rate results in improved audio quality. The voice bits are transmitted using two
timeslots of the 6 timeslot frame. This allows the use of the 8.0 kbs VSELP vocoder,
which increases the voice sampling rate to improve tone and richness. The iDEN
system also uses forward error correction to reduce corrupted bits in the voice
transmissions. This results in improved audio quality even in weak signal and
interference areas. This 3:1 interleave is used in interconnect calls.

Doubling the number of timeslots per call reduces the traffic carrying capacity of the
system. The timeslot allocation methods provides flexibility for the operator to
balance the requirements for superior audio quality against a reduction in capacity
and support for other services. The breakdown on 3:1 interleave voice channels is:
- Embedded signalling = 0.533333 kbps (24 bits per slot)
- FEC = 6.755555 kbps (304 bits per slot)
- Voice = 8 kbps (360 bits per slot)
- Total = 15.28888 kbps

Both 3:1 and 6:1 voice channels use the same transmission unit defined in the
Layer 2 RF Interface protocol. The main difference is that 3:1 interleave slots arrive
twice as often as 6:1 interleave slots. The method used will affect RF optimization
and planning.

Time Division Duplex:
To further optimize resources, the discrete timeslots in conversation are divided and
offset so transmit (Tx) and receive (Rx) control and voice information can share
resources. Both the uplink to the network and the downlink to the mobile can share
timelsots during interconnect calls. Interconnect calls use two carriers. The Mobile
Transmit and Receive frequencies are separate. With Time Division Duplex the
mobile dynamically shifts frequencies to send and receive voice, data and signalling
information.

Time Division Duplex (TDD) further reduces networkâ??s packet overhead and
eliminates the need for RF duplexer on the MS. To reduce errors in propagation
delay and allow the mobile to re-tune, the radio linkâ??s mobile receive packet is offset
ahead of the mobile transmit packet.

 

That's great info, thanks.